With excellent properties and easy melt molding, polyamides are broadly applied to materials for clothes, fibers for industrial materials, engineering plastics, and the like. Recently, polyamides used in the field of transparent components for industrial devices, mechanical, electrical, electronic and automobile parts, and optical materials such as glasses or lenses are required to have further improved properties and functions. Particularly, there is a need for development of polyamides that have further improved properties in terms of transparency, color, and mechanical strength.
Generally, polyamide resins are prepared through polycondensation of a dicarboxylic acid and a diamine. For example, Patent Document 1 (JP2009-203422A) discloses a continuous preparation method of a polyamide resin, in which a dicarboxylic acid having a certain structure and a diamine are continuously supplied to a polymerization reactor to produce a low-order condensate (polyamide prepolymer), which in turn is continuously supplied to a twin-screw extruder to produce a resin having a high polymerization degree.
However, the polyamine resin prepared by the method disclosed in Patent Document 1 fails to exhibit sufficient levels of properties in terms of transparency, color and mechanical strength, and is thus required to be further improved.
The inventors of the present invention have conducted research to solve the problems as mentioned above. From the research, the inventors completed the present invention based on the finding that a polyamide resin prepared to have a high degree of polymerization through solid polymerization of a low-order condensate, prepared by polycondensation of a dicarboxylic acid having a certain structure and a diamine in the presence of a phosphorus compound, exhibited significant improvement in terms of various properties, particularly, transparency.